It is well known in the art how to extract coal from deeply buried coal deposits. One method widely practiced is the so called "room and pillar" technique wherein the coal is extracted from the rooms and is left in place in the pillars. Size of each remnant coal pillar is dictated in part by the weight of the overburden which in itself may vary widely over the coal deposit in mountainous terrain. Lengths and widths of the various rooms are dictated in part by the hazards of roof fall, while the height of the room is generally controlled by the thickness of the coal seam. In some cases the thickness of the coal seam is greater than the efficient capacity of existing mining equipment and a portion of the coal seam is left unmined in the roof and in the floor. In such cases it is not uncommon to find examples where less than 50% of the coal in place has been removed when mining is completed and the mine abandoned.
The creation of void spaces underground induces significant stresses in the overburden and concentrates vertical loads in the remnant pillars. Coal, being a non-homogenous rock, inherently introduces uncertainties as to its vertical load carrying capabilities in any given location. Further, vertical load distribution is uneven among massive barrier pillars on the periphery of the mine (or around mine shafts) and each remnant pillar. It is not uncommon to find cases where the vertical load imposed on a particular remnant pillar exceeds the compressive strength of the coal, resulting in bursting of the pillar and shifting additional vertical loads to adjacent remnant pillars which also may burst. The result is a downwarping of the overburden, which in severe cases can cause tension cracks opening up from the mine workings through the overburden on to the surface of the ground above the mine.
While the tensional cracks tend to be in a near vertical alignment on the periphery of the downwarped overburden, compressive forces are predominent near the upper center of the downwarped area and normally cause buckling of the earth's crust. Any man-made structures in the path of these shifts in the earth's surface will be substantially damaged. Such shifting is commonly called subsidence.
Subsidence effects at the surface of the ground may be noticeable during the course of mining. In other cases the subsidence effects may not be apparent for many years after the mine is abandoned. Subsidence cracks can be several feet wide at the surface and pose grave hazards if left unattended. Hazards to people and animals are obvious. If the abandoned mine happens to be above the normal water table, a potential fire hazard also exists if one crack serves as an air intake and another crack serves as a chimney. Filling the cracks with inert material may correct the hazardous situation although there is no assurance that another crack will not appear without warning. The potential threat of additional subsidence can be eliminated by filling the void space underground, a practice that generally is more costly than the value of the coal originally removed. Or the mine may be reopened, if it is safe to do so, and the remnant pillars removed by further mining. Generally, abanoned mines fall far short of meeting modern day safety requirements and the cost of upgrading the old mine may be disproportionate to expected revenues from the coal recovered from the remnant pillars.
In many cases the lingering perils of subsidence over the years can be substantially foreshortened and effectively controlled by consuming the remnant pillars in situ, using methods disclosed in the instant invention together with methods taught in U.S. Pat. Nos. 3,987,852; 3,952,802 and 3,948,320; U.S. Pat. application Ser. Nos. 619,562 filed Oct. 6, 1975, now U.S. Pat. No. 4,010,801, and 665,128 filed Mar. 8, 1976, now U.S. Pat. No. 4,018,481; all of the instant inventor.
By consuming the remnant pillars in situ the roof of the mine can be lowered in a reasonably uniform manner until the roof and the floor substantially coincide, thereby ending the threat of further subsidence. The necessity of subjecting personnel to the hazards of old underground workings also is eliminated. Further, much of the coal remaining in place, including that in the roof and in the floor, can be converted to useful products such as low BTU fuel gas, synthesis gas, mixed coal chemicals and the like.
To accomplish the planned results the old mine workings must be sealed and remain sealed so that the underground chambers can be converted into pressurized reaction zones. Then the in situ techniques of gasification, liquefaction and pyrolysis can be employed to convert remaining coal into commercial products.